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Date:   Fri, 11 Nov 2022 17:27:20 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     "Michael Kelley (LINUX)" <mikelley@microsoft.com>
Cc:     Wei Liu <wei.liu@kernel.org>,
        Nuno Das Neves <nunodasneves@linux.microsoft.com>,
        Tianyu Lan <Tianyu.Lan@microsoft.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "iommu@lists.linux.dev" <iommu@lists.linux.dev>,
        Sunil Muthuswamy <sunilmut@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>,
        "dwmw2@infradead.org" <dwmw2@infradead.org>,
        "joro@8bytes.org" <joro@8bytes.org>,
        "will@kernel.org" <will@kernel.org>
Subject: Re: [PATCH] iommu/hyper-v: Allow hyperv irq remapping without x2apic
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On Fri, Nov 11, 2022 at 04:55:22PM +0000, Michael Kelley (LINUX) wrote:
> From: Wei Liu <wei.liu@kernel.org> Sent: Friday, November 11, 2022 8:33 AM
> > 
> > Hi Tianyu
> > 
> > On Wed, Nov 09, 2022 at 11:07:33AM -0800, Nuno Das Neves wrote:
> > > If x2apic is not available, hyperv-iommu skips remapping
> > > irqs. This breaks root partition which always needs irqs
> > > remapped.
> > >
> > > Fix this by allowing irq remapping regardless of x2apic,
> > > and change hyperv_enable_irq_remapping() to return
> > > IRQ_REMAP_XAPIC_MODE in case x2apic is missing.
> > >
> > 
> > Do you remember why it was x2apic only?
> > 
> > We tested this patch on different VM SKUs and it worked fine. I'm just
> > wondering if there would be some subtle breakages that we couldn't
> > easily test.
> > 
> > Thanks,
> > Wei.
> 
> My recollection is that originally Hyper-V provided the x2apic in the
> guest only when the number of vCPUs exceeded 255, and that was
> the only case where IRQ remapping was needed.  The intent was to
> not disturb the case where # of vCPUs was < 255 and the xapic is used.
> I don't remember there being any potential for subtle breakages.

Thanks for the information.

> 
> I think more recent versions of Hyper-V now provide the x2apic
> in the guest in some cases when # of vCPUs is < 255.
> 

On Azure the default for AMD SKUs is still xapic unless the number of
VCPUs exceeds 2XX (can't remember the exact number -- maybe it is 255).

Nuno, can you list the tests you've done? They will need to cover Linux
running as a normal guest on Azure and Hyper-V.

Thanks,
Wei.


> Michael
> 
> > 
> > > Tested with root and non-root hyperv partitions.
> > >
> > > Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
> > > ---
> > >  drivers/iommu/Kconfig        | 6 +++---
> > >  drivers/iommu/hyperv-iommu.c | 7 ++++---
> > >  2 files changed, 7 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > > index dc5f7a156ff5..cf7433652db0 100644
> > > --- a/drivers/iommu/Kconfig
> > > +++ b/drivers/iommu/Kconfig
> > > @@ -474,13 +474,13 @@ config QCOM_IOMMU
> > >  	  Support for IOMMU on certain Qualcomm SoCs.
> > >
> > >  config HYPERV_IOMMU
> > > -	bool "Hyper-V x2APIC IRQ Handling"
> > > +	bool "Hyper-V IRQ Handling"
> > >  	depends on HYPERV && X86
> > >  	select IOMMU_API
> > >  	default HYPERV
> > >  	help
> > > -	  Stub IOMMU driver to handle IRQs as to allow Hyper-V Linux
> > > -	  guests to run with x2APIC mode enabled.
> > > +	  Stub IOMMU driver to handle IRQs to support Hyper-V Linux
> > > +	  guest and root partitions.
> > >
> > >  config VIRTIO_IOMMU
> > >  	tristate "Virtio IOMMU driver"
> > > diff --git a/drivers/iommu/hyperv-iommu.c b/drivers/iommu/hyperv-iommu.c
> > > index e190bb8c225c..abd1826a9e63 100644
> > > --- a/drivers/iommu/hyperv-iommu.c
> > > +++ b/drivers/iommu/hyperv-iommu.c
> > > @@ -123,8 +123,7 @@ static int __init hyperv_prepare_irq_remapping(void)
> > >  	const struct irq_domain_ops *ops;
> > >
> > >  	if (!hypervisor_is_type(X86_HYPER_MS_HYPERV) ||
> > > -	    x86_init.hyper.msi_ext_dest_id() ||
> > > -	    !x2apic_supported())
> > > +	    x86_init.hyper.msi_ext_dest_id())
> > >  		return -ENODEV;
> > >
> > >  	if (hv_root_partition) {
> > > @@ -170,7 +169,9 @@ static int __init hyperv_prepare_irq_remapping(void)
> > >
> > >  static int __init hyperv_enable_irq_remapping(void)
> > >  {
> > > -	return IRQ_REMAP_X2APIC_MODE;
> > > +	if (x2apic_supported())
> > > +		return IRQ_REMAP_X2APIC_MODE;
> > > +	return IRQ_REMAP_XAPIC_MODE;
> > >  }
> > >
> > >  struct irq_remap_ops hyperv_irq_remap_ops = {
> > > --
> > > 2.25.1
> > >